Using hybrid, inbred, and random bred mouse strains we will investigate (1) whether the relationship of increased maternal age to hyperdiploidy in mouse fetuses reflects a relationship existing in mature mouse ova or is secondary to a decreased ability to reject hyperdiploid embryos, and (2) when the meiotic error or errors occur that cause heteroploidy in mouse embryos. For the latter, oocytes matured in vivo and in vitro to metaphase I and metaphase II and pronuclear zygotes will be used. Finally, the effect of such variables as maternal age, the recent use of birth control pills, and the method inducing oocyte maturation on the incidence of heteroploidy in maturing human ova will be examined. Human ova in the germinal vesicle stage for in vitro maturation will be gotten by mincing ovaries obtained at elective gynecologic surgery.